1. Field or the Invention
The present invention generally relates to a multilayer dielectric optical structure that selectively reflects a predetermined wavelength of electromagnetic radiation. More particularly, this invention relates to the optical structure whose change of optical property is activated by temperature.
2. Background of the Invention
Glass windows are widely used in residential and commercial buildings for the purpose of natural light collection as well as for aesthetic reasons. However, glass windows, as they are generally a thin and transparent barrier separating the interior for example an office space to the outside environment, can readily exchange heat with the outside environment via two paths: direct heat exchange due to thermal motions of air, and passage of electromagnetic radiation. The reduction of direct heat exchange due to thermal motion between an interior and external environment are generally always preferred. For electromagnetic radiation, there are two major contributions as far as heat exchange is concerned: the long wavelength radiation due to blackbody radiation of objects near room temperature, and the solar electromagnetic radiation. Similar to heat exchange due to thermal motion, the transfer of blackbody radiation due to objects near room temperature are generally not preferred as they present a heat loss due to interior in colder days (to colder environment) and heating of interior on hotter days from hotter external environment. However, heating due to solar radiation is a different matter. Although visible light in general are preferred to transmit through the windows to interior, the near infrared spectrum of solar radiation or the heat component of the solar spectrum are desirable only in colder days, and on a hot summer day, rejection of the solar heat is very much desirable.
To reduce the heat exchange due to thermal motion, double pane glass windows with an air gap or inert gas filled gaps or triple pane glass windows are often used. However, these windows do not reduce or increase solar heat gain, as they absorb or reflect only a very small and fixed amount of electromagnetic radiation in the visible and infrared range that are essentially purely due to Fresnel reflections.
Current techniques employed in reducing the passage of electromagnetic radiation via glass windows include the technique of coating a very thin layer or layers of material, for example, silver and silver nitride layers, that behaves nearly as a metal mirror for wavelengths of about 10 μm? electromagnetic radiation. Such coated windows are commonly known as low e or low emissivity windows, and reflect the long wavelength electromagnetic radiations back to the environment or interior of a building. Such coatings increase the heat insulation properties of windows at all temperatures.
Selective reflection and absorption of near infrared radiation is a mature technology with great commercial success. An example is Solarban 70XL coatings produced by PPG Industries, Inc. Such coatings block most of near infrared and partially visible light constantly, both in colder and warmer days.
However, it is desirable to have a window or a film that have high transmission of visible light; high transmission of infrared radiation when temperature is low; transition to high reflection of infrared radiation when temperature reaches certain level; the transition temperature is at room temperature range; the switching is automatic depending on the temperature; the window or film can be inexpensively mass producedand non-toxic.
3. Prior Arts
U.S. Pat. No. 3,279,327 to Ploke disclosed a multilayer optical filter for selectively reflecting infrared radiation while allowing visible light to transmit by means of interference.
U.S. Pat. No. 4,229,066 to Rancourt et al, disclosed a multilayer stack which is reflecting in the infrared and transmitting at shorter wavelengths. The stack is formed of a plurality of layers of high and low index materials with alternate layers being formed of materials having a high index of refraction and the other layers being formed of materials having a low index of refraction.
U.S. Pat. No. 3,711,176 to Alfrey et al, disclosed a multilayer films of polymer materials with sufficient mismatch in refractive indices, these multilayer films cause constructive interferences of light. This results in the film transmitting certain wavelengths of light through the film while reflecting other wavelengths.
U.S. Pat. No. 3,790,250 to Mitchell disclosed a system, where the conductivity of a light absorbing semiconductor varies with the temperature, and inversely, the light absorption or attenuation level is controlled by the temperature. The disclosed system show a light absorption of about 80% at 80° C. and the absorption is reduced to about 15% at room temperature. This system is not useful for present application because its temperature dependence changes slowly over a wide range of temperature.
U.S. Pat. No. 4,307,942 to Chahroudi disclosed a solar control system where various structure consisting of porous layers absorb the solvents or repel the solvents depending on the temperature and the structures change their optical properties from transparent to solar radiation at low temperatures to a metallic surface or a dielectric mirror to reflect solar radiation of predetermined wavelengths. A significant difficulty in implementing such a device, aside from any performance issues, is that there must be a significant reservoir to hold such solvents.
U.S. Pat. No. 4,401,690 to Greenberg disclosed a method for making thin films of vanadium oxide possessing such a transition with depressed transition temperature of 25° C. to 55° C., approaching but not quite the transition temperature needed in order that the material to be useful. In addition, vanadium oxides in temperatures below or above the transition temperature absorb a significant portion of visible light, which makes the technology less attractive for window applications.
U.S. Pat. No. 6,049,419 to Wheatley disclosed a dielectric multilayer structure consisting of birefringent polymer layers that will reflect at least one polarization of predetermined wavelengths. However, the optical properties including its reflectance will not change with the temperature, and the resulting optical structure will reject solar energy on a warmer summer day, which is desirable, as well as on a cold winter day if so designed, which is not desirable.
U.S. invention disclosure No. H001182 by Spry disclosed an optical filter structure using a material that has a ferroelectric phase to a non ferroelectric phase transition upon changing in temperature and another optically clear material that does not have the phase transition. The resulting optical filter structure can selectively block radiation of a predetermined wavelength, as the refractive index of the phase changing layer changes as temperature change. However, the transition temperature of ferroelectric materials occur at about 120° C., the induced index of refraction change is about 0.03, and as both layers are optically isotropic in the reflection mode at high temperature, the device will only reflect a nearly normal incident single wavelength light at very high temperatures, and it will require a large number of layers, greater than 5000 to achieve significant reflection across a broadband of near infrared radiations, therefore that will not be applicable for adjusting solar energy control at room temperature range.
Accordingly, the unfulfilled needs still exists in this art for a window or an optical film that have high transmission of visible light; high transmission of infrared radiation when temperature is low; transition to high reflection of infrared radiation when temperature reaches certain level; the transition temperature is at room temperature range; the switching is automatic depending on the temperature; the window or film can be inexpensively mass produced; the material used for such windows or films is non-toxic.